It is proposed to study the stereochemical changes that occur on ligation of dioxygen and other small molecules to metalloporphyrin systems that are highly congruent models for the heme centers in hemoproteins. Emphasis will be on the determination by diffraction methods of the stereochemistry of the models themselves and of their adducts with molecular oxygen, carbon monoxide, nitric oxide, isocyanides and other small molecules. Further it is proposed to investigate more thoroughly the nature of the porphyrin core, including its stereochemical flexibility and the nature of its bonding, by determining the structures of selected model systems that involve reduced porphyrins, e.g., chlorins or bacteriochlorins or isobacteriochlorins. And as suitable crystals arise, extensive low-temperature diffraction data will be collected for analysis of the bonding electron distributions in these metalloporphyrins and reduced systems. Correlation of physical and chemical properties with the bonding characteristics of these model systems is necessary for a more thorough understanding of the heme sites in the hemoproteins. In particular, structural data on these model systems are essential for an understanding of the electronic and steric factors that affect their ligation by small molecules, such as dioxygen; such information on the model systems is necessary for our understanding of structure-function relationships, including allosteric properties, in the hemoproteins.